A control apparatus for varying steering power in a power steering unit in accordance with the running conditions of a vehicle, i.e., the steering conditions, etc., is arranged such that the steering angle is generally used as one of the steering conditions. A quantity of rotation of a handle shaft is detected by a steering angle sensor so as to rotate a steering shaft by a quantity corresponding to a detection signal of the sensor. This is disclosed, for example, in Japanese Patent Application (OPI) No. 60967/82 (the term "OPI" as used herein means an unexamined published application).
Conventionally, a potentiometer is used as the sensor for detecting a steering angle of a vehicle. Since the range of the steering angle corresponds to several revolutions of a handle shaft, there is a relationship as shown in FIG. 1B between the output voltage V.sub.out of a potentiometer 1 shown in FIG. 1A and a steering angle .theta.. In FIG. 1B, the shaded portions show the ranges of steering angle .theta. corresponding to the state where a slider member 1a of the potentiometer 1 is positioned off of a resistor 1b. In these ranges, the output terminal of the potentiometer 1 is in a floating state because it is separated from both a source terminal +Vcc and an earth terminal E.
In a steering angle sensor of this kind, on the other hand, a low-pass filter 2 comprising a resistor R and a capacitor C as shown in FIG. 2, is provided in a circuit that is supplied with the output of the potentiometer 1. Accordingly, if the slider member 1a is in a floating state, the capacitor C is discharged through an input impedance Z.sub.i of a circuit portion 3 in the next stage of the low-pass filter 2. Consequently, a voltage V.sub.in supplied from the filter 2 to the succeeding stage circuit portion 3 varies as shown in FIG. 3A.
When the slider member 1a of the potentiometer 1 reaches the earth terminal E, the capacitor C is rapidly discharged through the resistor R of the low-pass filter 2 so that the voltage V.sub.in is lowered steeply as shown in FIG. 3B. If the slider member 1a is moved beyond the source terminal +Vcc from the earth terminal, the capacitor C is suddenly charged so that the voltage V.sub.in rises steeply as shown in FIG. 3C. The variations in the voltage V.sub.in due to the charging and discharging of the capacitor C in the low-pass filter 2 cause the voltage V.sub.in not to correspond to an actual steering angle. Accordingly, the use of such an output signal of the filter as a signal representing the steering angle causes errors in the foregoing control of steering power. That is, it becomes impossible to judge whether the variation in the output voltage of the steering angle sensor is caused by the steering operation or by the charging and discharging of the capacitor.
To solve this problem, it has been proposed to provide a gearing mechanism between the potentiometer and the handle shaft to limit the maximum revolutional angle of the potentiometer to within 360 degrees. The handle shaft is rotated by several turns within this 360 degree range of revolutional angle. In this case, however, problems occur in the resolution of the steering angle and accuracy is reduced due to backlash of the gearing mechanism.
It is therefore an object of the present invention to solve the foregoing problems in the prior art.
Another object of the present invention is to provide a method of processing a signal of a steering angle sensor in which when an output voltage of a potentiometer is detected as a signal representing a steering angle, it is possible to improve the resolution of steering angle.
A further object of the present invention is to prevent a signal that does not correspond to an actual steering angle due to a noise filter from being erroneously detected in a range of steering angle where a slider member of the potentiometer is in a floating state.
These and other objects are attained by a method for generating a steering angle signal for a motor vehicle by processing a signal produced by a steering angle sensor to eliminate erroneous steering angle signal values caused by a floating state of a potentiometer included in the sensor to output the steering angle signal value corresponding to the rotational angle of a rotatable steering handle shaft when the erroneous signal value occurs at specific angular positions of the handle shaft when the potentiometer is in the floating state, the method comprising the steps of determining the current steering angle signal value outputted by the potentiometer, determining if the rotational angle of the handle shaft is within a selected range of rotational angles, calculating an estimated steering angle signal value if the rotational angle of the handle shaft is determined to be within the selected range of rotational angles, calculating a first difference value corresponding to the difference between the current steering angle signal value and the estimated steering angle signal value, setting the steering angle signal to the current steering angle signal value if the first difference value is less than a predetermined difference value, and setting the steering angle signal to a predetermined angle signal value if the first difference value is greater than an equal to the predetermined difference value whereby the steering angle signal is set to the current steering angle signal value if the potentiometer is not in the floating state and is set to the predetermined angle signal value if the potentiometer is in the floating state.